As the power sources for portable electronic equipment, non-aqueous secondary batteries that have a high capacity and are light in weight, namely, lithium ion secondary batteries, are widely used in recent years. A non-aqueous secondary battery has a porous separator made of resin for electrically insulating positive and negative electrodes from each other and for retaining a non-aqueous electrolyte. The resin separator melts when the battery temperature is too high, and clog the micropores thereof. This prevents the migration of ions through the electrolyte, and hence the safety of the battery is maintained. Such function of the separator is called shut-down function. A resin separator is usually made of a resin susceptible to thermal deformation such as polyolefin resin.
When a battery is charged to have more than its nominal capacity, in other words, when overcharge is performed, the battery temperature might be very high. In such case, the separator's shut-down function is expected to work. However, when a current during overcharge is relatively large, the battery temperature might increase suddenly and the separator sometimes shrinks to smaller than the positive and negative electrodes in width. This allows the positive and negative electrodes whose reactivity has been increased to contact with each other, which might accelerate overheating.
In an attempt to overcome the above problem while the conventional shut-down function is maintained, Japanese Laid-Open Patent Publication No. 2001-266828 (Patent Document 1) proposes a separator comprising an inorganic porous membrane containing inorganic powders and having high thermal resistance and an organic porous membrane made of a polyolefin resin alternately laminated. The organic porous membrane is produced by mixing a polyolefin resin, inorganic powders and an appropriate amount of an organic plasticizer, which is kneaded with heating and formed into a sheet, from which the inorganic powders and the organic plasticizer are extracted. In this production method, the organic porous membrane will have a large micropore volume ratio (porosity), and therefore it becomes brittle in structure. As such, considering the battery producing process, particularly the ease of handling during the step of spirally winding the positive and negative electrodes with the separator interposed therebetween, the separator including the inorganic porous membrane and the organic porous membrane is required to have a very large thickness, namely, not less than 40 μm. Accordingly, it is difficult to achieve a battery having a capacity as high as that of a battery using a conventional separator with a thickness of about 15 to 30 μm. In the case of Patent Document 1, the capacity ratio of high rate discharge to low rate discharge will also be low.
As a means to enhance safety during an abnormal increase of battery temperature due to overcharge while maintaining a high capacity, there is a method in which a slurry containing a metal oxide filler and a resin binder is applied onto the surface of an electrode plate by an ordinary printing method, followed by drying with hot air to form a porous membrane, for example. This method has the advantage of low material cost and low production cost. Moreover, even if the battery is short-circuited due to the penetration of a nail or the like, because the metal oxide filler has high thermal resistance, the extension of the short-circuited area will be prevented. Accordingly, the above-described method ensures a high level of safety. When using only a conventional separator, the short-circuited area is extended along with heat generation, which might contribute to further heat generation.
In the case of the porous membrane containing a metal oxide filler and a resin binder, however, because the metal oxide filler contributes to maintain porosity of the membrane even during an abnormal increase of the battery temperature, although the resin binder melts, the migration of ions through the electrolyte cannot be prevented, which means the shut-down function might not work properly.